Investigation of in-liquid ordering mediated transformations in Al-Sc via ab initio molecular dynamics and unsupervised learning

Abstract

Scandium is well known to produce grain refinement in Al-based alloys, and its potency is generally attributed to intermetallic ${\mathrm{Al}}_{3}\mathrm{Sc}$ formation within liquid phase. However, the influence of Sc atoms and ${\mathrm{Al}}_{3}\mathrm{Sc}$ on the local structure of the surrounding melt, and subsequent nucleation remains unclear. Towards that end, we have probed structural changes in three bulk compositions, i.e., $\mathrm{Al}\text{\ensuremath{-}}x\mathrm{Sc}$ $(x=0, 0.4, 1.0 \mathrm{at}.%)$, and near liquid-$\mathrm{Al}/{\mathrm{Al}}_{3}\mathrm{Sc}$ interfacial regions using ab initio molecular dynamics. In-liquid ordering was determined using unsupervised learning techniques, i.e., structural fingerprinting, dimensionality reduction, and cluster analysis. Sc atoms ordered the surrounding liquid Al atoms by forming Sc-centered polyhedrons, while liquid-$\mathrm{Al}/{\mathrm{Al}}_{3}\mathrm{Sc}$ interface manifested planar ordering that resembled ${{100}}_{fcc\ensuremath{-}\mathrm{Al}}$. Both structures were geometrically persistent but constitutionally transient, i.e., they exchanged Al atoms with the surrounding liquid. This behavior was rationalized on the basis of their mixed metallic and covalent bond character. At a lower temperature, ${{100}}_{fcc\ensuremath{-}\mathrm{Al}}$ interfacial planes heteroepitaxially nucleated equilibrium $fcc\text{\ensuremath{-}}\mathrm{Al}$, while Sc-centered polyhedrons sequentially formed metastable $hcp$- and $bcc\text{\ensuremath{-}}\mathrm{Al}$. Using our simulations and extant experimental reports, we postulate two transformation pathways during Al-Sc solidification: (i) $\mathrm{Sc}\ensuremath{-}centered\phantom{\rule{4pt}{0ex}}polyhedrons\ensuremath{\rightarrow}{\mathrm{Al}}_{3}\mathrm{Sc}\ensuremath{\rightarrow}liquid\ensuremath{-}Al/A{l}_{3}Sc\phantom{\rule{4pt}{0ex}}interfacial\phantom{\rule{4pt}{0ex}}ordering\ensuremath{\rightarrow}fcc\ensuremath{-}\mathrm{Al}$; and (ii) $\mathrm{Sc}\ensuremath{-}centered\phantom{\rule{4pt}{0ex}}polyhedrons\ensuremath{\rightarrow}hcp\ensuremath{-}\mathrm{Al}\ensuremath{\rightarrow}bcc\ensuremath{-}\mathrm{Al}\ensuremath{\rightarrow}fcc\ensuremath{-}\mathrm{Al}$. The two in-liquid ordered structures provide an atomistic basis for the potency of Sc element, and, also, serve as possible structural metrics for designing novel Al-based alloys.